I am currently designing a sample holder that will be used in liquid helium.
Has anyone experience with exposing 3D printed materials to low temperatures? Generally, the CTE is important which is rather large for most plastics. Maybe the alu filled nylon might work? Just wondering if anyone has experience with this.

Nylon and cold don't go well together, looses its elasticity and becomes fragile. What's wrong with a stainless steel material for your application ? SS goes very well with lowtemp such as liquid helium.

Ok, looking at the material density there is probably not a very high aluminum dust content so this is not likely going to attribute much to the CTE. There is also some strength needed so stainless steel might be a better way to go, I just wanted to explore the least expensive option first..

While the s/steel here may be watertight I don't know that I would be sticking it in liquid helium. The printing process (I could be wrong here) leaves microscopic tunnels of empty space. It's been a loooong time since I was in a lab but I don't think that's a good thing.

I think the problem is with the expanding of the liquid when taken out of the cryostat and going to vapor state. However, the material should be strong enough to withstand that, I'd imagine.
Does anybody actually have a density number for printed stainless steel? That may give an indication of porosity.

To avoid this "microcracks" in the SS you can always try to apply a heat treatment on the part and polish it as much as possible. I am following some mechanical tests on different 3D printed metal alloys and the data are completely different if after manufacturing treatments are made on the part.
I am not saying this will make the part completely liquidproof, just an idea.

Thanks for the advice, I can imagine heat treating has major impact on metal properties.
In the end, I discovered that the SS is magnetic, which makes it unfit for our purposes; I have ordered the part in aluminum from incodema.